The invention concerns a process for producing flame-retardant polyamides.
Synthetic polyamides (PA) are used in a variety of applications in many areas of the industry and for everyday consumption. This is due mainly to the good processing properties and the possibility of tailoring these polymers to the application. At present, just under 90% of polyamide consumption consists of the standard types polyamide 6 (poly-.epsilon.-caprolactam) and polyamide 66 (polyhexamethyleneadipamide); polyamide 11 (polyundecaneamide), polyamide 12 (poly-.epsilon.-laurinlactam), polyamide 610 (polyhexamethylenesebacamide) and polyamide 612 (polyhexamethylenedodecaneamide) and copolyamides account for the remaining 10%. More than 80% of worldwide polyamide production is processed to fibers and fabrics; just under 20% is used in industrial applications, in particular in automotive engineering, the electronics industry, the packaging sector and construction of machinery and equipment. The good mechanical properties often required industrially are achieved with fiber reinforcement or mineral fillers. In the field of electrical engineering, the use of polyamides has been successful because of their high insulation resistance, good tracking resistance and solvent resistance as well as good thermo-mechanical properties, in particular for insulation and switch parts, solenoid valves, busbars, cable mounts, coil bodies, plug connectors, and casings.
Although polyamides are self-extinguishing according to some test methods, they lose this property after the addition of fillers such as glass fibers or pigments. For numerous applications in electrical engineering and in automotive engineering, however, reinforced, flameproof polyamide is needed. The flameproofing should offer enough time to rescue people and valuables in the event of a fire.
At the present time, mainly organic halogen compounds and red phosphorus are used as flameproofing agents. The halogen compounds are mainly chlorinated or brominated hydrocarbons, which are often combined with zinc compounds or antimony trioxide, the latter of which has a synergistic effect but has been found to be carcinogenic in animal experiments. Halogen compounds have the disadvantage that they release highly corrosive and highly toxic degradation products such as hydrogen chloride and hydrogen bromide in a fire and they cause heavy production of smoke; they also reduce the toughness and tracking resistance of polyamides. Red phosphorus is usually used in encapsulated form. Despite the encapsulation, however, there is the danger of phosphorus fires at high processing temperatures. This can lead to increased wear on the processing machines and even explosions as a result of disproportionation to phosphine and phosphates. Another disadvantage is the poor electrical corrosion property of polyamide materials finished with red phosphorus to be flame-retardant, besides their dark color.
To avoid the disadvantages associated with halogen compounds and red phosphorus, there have been attempts for several years to develop flameproofed polyamides without such flameproofing agents. For example, the use of nitrogen compounds such as dicyanodiamide (German Offenlegungsschrift No. 3,909,145), melamine and melamine salts (German Offenlegungsschrifts Nos. 3,609,341 and 4,141,861) and melamine adducts (German Offenlegungsschrift No. 3,722,118) has been proposed. To achieve adequate flame retardancy, in particular with glass fiber-reinforced materials, however, very high filler levels are required, which have a negative effect on the mechanical properties. Magnesium hydroxide, which has also been proposed (Kunststoffe, vol. 80 (1990) pages 1107-1112), also causes a weakening of the mechanical strength, when used in the required high concentrations; the release of water, which begins at the processing temperature, also causes bubbles to form. For partially aromatic polyamides, the use of high concentrations of polyphosphonates has also been proposed (German Offenlegungsschrift No. 36 13 490). However, the flame retardancy achieved in that way is inadequate at high levels of glass fiber filling; moreover, the mechanical properties of the polyamides are severely impaired.
Furthermore, it has already been proposed that polyamide synthesis be performed in the presence of compounds that are incorporated into the polymer chain during polymerization. Thus, for example, the use of N-phosphonates and N-phosphates of .epsilon.-caprolactam has been recommended for polymerization of .epsilon.-caprolactam (see Journal of Applied Polymer Science, vol. 47 (1993) pages 1185-1192). In addition, the synthesis of phosphorus-containing copolymers, e.g., by reacting 3,3'-diaminodiphenyl phosphine oxide with 1,3-phenyleneisophthalamide, and the use of these copolymers as flameproofing agents have been recommended (see Journal of Polymer Science, Part A, Polymer Chemistry, vol. 30 (1992) pages 2521-2529). Apart from the great expense required in these cases, the resulting flame retardancy is inadequate for industrial applications.